The 'warping constant' Iw is used in the design of steel beams for:
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Deflection of beams under vertical load
Lateral-torsional buckling — warping resistance Iw adds to torsional stiffness in the Mcr formula
Punching shear in flat slabs
Calculation of moment of inertia about centroidal axis
Correct Answer
B. Lateral-torsional buckling — warping resistance Iw adds to torsional stiffness in the Mcr formula
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Warping constant Iw: geometric property relevant to lateral-torsional buckling of open sections (I-beams, channels). For I-section: Iw = Iy×h²/4 (h = distance between flange centroids). In LTB critical moment: Mcr = π/L √(EIy×(GIt + π²EIw/L²)) — warping resistance adds to pure torsional resistance (GIt). Warping torsion dominant for short segments; St. Venant (GIt) dominant for long.
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